CHAPTER 9 CELLULAR RESPIRATION: HARVESTING CHEMICAL ENERGY Section A: The Principles of Energy Harvest 1. Cellular respiration and fermentation are catabolic, energy-yielding pathways 2. Cells recycle the ATP they use for work 3. Redox reactions release energy when electrons move closer to electronegative atoms 4. Electrons fall from organic molecules to oxygen during cellular respiration 5. The fall of electrons during respiration is stepwise, via NAD + and an electron transport chain
Introduction Living is work. To perform their many tasks, cells require transfusions of energy from outside sources. In most ecosystems, energy enters as sunlight. Light energy trapped in organic molecules is available to both photosynthetic organisms and others that eat them. Fig. 9.1
1. Cellular respiration and fermentation are catabolic, energy-yielding pathways Organic molecules store energy in their arrangement of atoms. Enzymes catalyze the systematic degradation of organic molecules that are rich in energy to simpler waste products with less energy. Some of the released energy is used to do work and the rest is dissipated as heat.
Metabolic pathways that release the energy stored in complex organic molecules are catabolic. One type of catabolic process, fermentation, leads to the partial degradation of sugars in the absence of oxygen. A more efficient and widespread catabolic process, cellular respiration, uses oxygen as a reactant to complete the breakdown of a variety of organic molecules. Most of the processes in cellular respiration occur in mitochondria.
Cellular respiration is similar to the combustion of gasoline in an automobile engine. The overall process is: Organic compounds + O 2 -> CO 2 + H 2 O + Energy Carbohydrates, fats, and proteins can all be used as the fuel, but it is traditional to start learning with glucose. C 6 H 12 O 6 + 6O 2 -> 6CO 2 + 6H 2 O + Energy (ATP + heat) The catabolism of glucose is exergonic with a delta G of - 686 kcal per mole of glucose. Some of this energy is used to produce ATP that will perform cellular work.
2. Cells recycle the ATP they use for work ATP, adenosine triphosphate, is the pivotal molecule in cellular energetics. It is the chemical equivalent of a loaded spring. The close packing of three negatively charged phosphate groups is an unstable, energy-storing arrangement. Loss of the end phosphate group relaxes the spring. The price of most cellular work is the conversion of ATP to ADP and inorganic phosphate (P i ). An animal cell regenerates ATP from ADP and P i by the catabolism of organic molecules.
The transfer of the terminal phosphate group from ATP to another molecule is phosphorylation. This changes the shape of the receiving molecule, performing work (transport, mechanical, or chemical). When the phosphate group leaves the molecule, the molecule returns to its alternate shape. Fig. 9.2
3. Redox reactions release energy when electrons move closer to electronegative atoms Catabolic pathways relocate the electrons stored in food molecules, releasing energy that is used to synthesize ATP. Reactions that result in the transfer of one or more electrons from one reactant to another are oxidationreduction reactions, or redox reactions. The loss of electrons is called oxidation. The addition of electrons is called reduction.
The formation of table salt from sodium and chloride is a redox reaction. Na + Cl -> Na + + Cl - Here sodium is oxidized and chlorine is reduced (its charge drops from 0 to -1). More generally: Xe - + Y -> X + Ye - X, the electron donor, is the reducing agent and reduces Y. Y, the electron recipient, is the oxidizing agent and oxidizes X. Redox reactions require both a donor and acceptor.
Redox reactions also occur when the movement of electrons is not complete but involve a change in the degree of electron sharing in covalent bonds. In the combustion of methane to form water and carbon dioxide, the nonpolar covalent bonds of methane (C-H) and oxygen (O=O) are converted to polar covalent bonds (C=O and O-H). Fig. 9.3
When these bonds shift from nonpolar to polar, the electrons move from positions equidistant between the two atoms for a closer position to oxygen, the more electronegative atom. Oxygen is one of the most potent oxidizing agents. An electron looses energy as it shifts from a less electronegative atom to a more electronegative one. A redox reaction that relocates electrons closer to oxygen releases chemical energy that can do work. To reverse the process, energy must be added to pull an electron away from an atom.
4. Electrons fall from organic molecules to oxygen during cellular respiration In cellular respiration, glucose and other fuel molecules are oxidized, releasing energy. In the summary equation of cellular respiration: C 6 H 12 O 6 + 6O 2 -> 6CO 2 + 6H 2 O Glucose is oxidized, oxygen is reduced, and electrons loose potential energy. Molecules that have an abundance of hydrogen are excellent fuels because their bonds are a source of hilltop electrons that fall closer to oxygen.
The cell has a rich reservoir of electrons associated with hydrogen, especially in carbohydrates and fats. However, these fuels do not spontaneously combine with O 2 because they lack the activation energy. Enzymes lower the barrier of activation energy, allowing these fuels to be oxidized slowly.
5. The fall of electrons during respiration is stepwise, via NAD + and an electron transport chain Cellular respiration does not oxidize glucose in a single step that transfers all the hydrogen in the fuel to oxygen at one time. Rather, glucose and other fuels are broken down gradually in a series of steps, each catalyzed by a specific enzyme. At key steps, hydrogen atoms are stripped from glucose and passed first to a coenzyme, like NAD + (nicotinamide adenine dinucleotide).
Dehydrogenase enzymes strip two hydrogen atoms from the fuel (e.g., glucose), pass two electrons and one proton to NAD + and release H +. H-C-OH + NAD + -> C=O + NADH + H +
This changes the oxidized form, NAD +, to the reduced form NADH. NAD + functions as the oxidizing agent in many of the redox steps during the catabolism of glucose. Fig. 9.4
The electrons carried by NADH loose very little of their potential energy in this process. This energy is tapped to synthesize ATP as electrons fall from NADH to oxygen.
Unlike the explosive release of heat energy that would occur when H 2 and O 2 combine, cellular respiration uses an electron transport chain to break the fall of electrons to O 2 into several steps. Fig. 9.5
The electron transport chain, consisting of several molecules (primarily proteins), is built into the inner membrane of a mitochondrion. NADH shuttles electrons from food to the top of the chain. At the bottom, oxygen captures the electrons and H + to form water. The free energy change from top to bottom is - 53 kcal/mole of NADH. Electrons are passed by increasingly electronegative molecules in the chain until they are caught by oxygen, the most electronegative.